In order to clean exhaust gases produced by internal combustion engines to remove particles, particularly diesel engines, particle filters located in an exhaust gas duct of the internal combustion engine can be employed, through which the exhaust gas is routed. Such a particle filter filters the particles out of the exhaust gas stream, whereby the particles remain in the particle filter. In order to maintain the continuing function of the particle filter, the latter must be regenerated once it has reached a certain loading level. To this end it is necessary to check the loading status of the particle filter.
For this purpose, it is advantageous to utilize the effect whereby the pressure difference between the exhaust gas inlet and the exhaust gas outlet of the particle filter, also referred to as differential pressure in the following, depends on the loading with particles. In order to measure the pressure difference differential pressure sensors are employed which are connected in parallel with the particle filter into the exhaust gas duct and are thus able to sense the drop in pressure in the exhaust gas stream across the particle filter, in other words the difference between the pressure directly upstream of the particle filter and the pressure directly downstream of the particle filter. In this situation and in the following the expressions “directly upstream” and “directly downstream” are understood to mean that no sections of the exhaust gas duct lie between the thus designated area in the exhaust gas duct directly upstream or downstream of the particle filter and the exhaust gas inlet or outlet of the particle filter, which cause a drop in pressure comparable to the drop in pressure at the particle filter, such that the differential pressure sensor in actual fact essentially senses the differential pressure generated by the particle filter.
To this end the differential pressure sensors have two gas connections which are also referred to in the following as high and low pressure connections. The high pressure connection is defined by the fact that the sensor delivers a sensor signal which represents a positive differential pressure when a greater pressure is present at the high pressure connection than at the low pressure connection. The designation high pressure connection or low pressure connection does not however imply in any way that high pressure, in other words a quantitatively high pressure, or low pressure, in other words a quantitatively low pressure, is applied to this connection.
The differential pressure sensors used do however exhibit properties which make it necessary for the high pressure connection of the differential pressure sensor to be connected to the exhaust gas duct upstream of the particle filter and the low pressure connection of the differential pressure sensor to be connected to the exhaust gas duct downstream of the particle filter.
Thus, although the differential pressure sensors used have a linear characteristic, in other words a linear relationship between the voltage of the sensor signal and the pressure difference present between the high and the low pressure connections, the offset does however drift, in other words the voltage of the sensor signal changes at a differential pressure of 0 bar over the course of time. Moreover, the measuring range for negative differential pressures in which a reliable measurement is possible is only very restricted.
Therefore, if when the differential pressure sensor is installed, for example during manufacture or in the event of a repair, it is connected with high and low pressure connections to the exhaust gas duct reversed, incorrect values will be supplied for the differential pressure sensed. Reliable detection of the loading status of the particle filter would not then be possible, with the result for example that the need for regeneration would not be reliably detected and the particle filter could then no longer adequately clean the exhaust gas.
It would now be conceivable, before starting up the internal combustion engine, if there is no pressure difference between the high and the low pressure connection, to determine the offset of the differential pressure sensor by sensing the differential pressure. On the basis of differential pressures which have been determined with the particular offset, it is then possible to ascertain whether or not reversed connections have been made.
However, it appears difficult to define the offset with sufficient accuracy before starting up the internal combustion engine. Only the time between turning on the ignition and starting the engine remains for determining the offset. This period of time can however be too short for determining the offset, for example if the driver turns the ignition key through the position in which the ignition is turned on, fully into the starting position. Moreover, the starter can place such a load on the vehicle electrical system that the differential pressure sensor and an evaluation facility for the signals from the sensor are not adequately supplied with power. Furthermore, for example in the case of very cold ambient conditions, the particle filter and the differential pressure sensor may be in a state which is not typical for operation and cannot therefore be used. Adaptation of the offset before starting up the internal combustion engine accompanied by detection based thereon of a reversed connection therefore appear less than appropriate.
For this reason it would be desirable to be able to detect or diagnose an incorrect connection of the differential pressure sensor, in other words a connection of the low pressure connection to the exhaust gas duct directly upstream of the particle filter and connection of the high pressure connection to the exhaust gas duct directly downstream of the particle filter, in a simple manner.